The present invention relates to radiation pyrometry, wherein the radiation densities of two different wave lengths are separately monitored and detected for obtaining an indication of temperature.
The so-called ratio pyrometers use the radiation from a common source, and photoelements or the like are provided to detect the radiation density of two different wavelengths. The resulting electrical signals are processed by forming their quotient or ratio which is indicative of temperature. This method is very advantageous for measuring the temperature of rather hot objects, possibly even moving objects, such as rolled ingots having a temperature of, say, between 700.degree. and 1200.degree. Centrigrade. The monitoring of welding seams requires responses even for still higher temperatures. The method outlined above and using two separate radiation components has certain advantages over the summary or integrating detection of radiation density (covering a wide range of the spectrum) or over the detection of the radiation density of a single wavelength. Any interference due to variable emission of the surface or any absorbtion in the radiation path will cause only little change in the result, because any influence that mathematically amounts to a multiplication of the density or intensity as detected, by the same "factor" as to each of the two components will cancel on formation of the ratio.
In one known apparatus of this type one uses photoelements "loaded" with a passive resistor of a few hundred Ohms to obtain voltages in the range of a few millivolts. The two photoelements are connected so that the respective voltages oppose, and together they establish one half of a bridge. This bridge is connected to the input of a null-amplifier whose output controlls an actuator. This actuator changes the load on one of the elements until the input voltage across the input of the amplifier is again zero. The output current or voltage of this high gain amplifier constitutes a measure for the radiation temperature.
The accuracy of this known equipment is, however, not sufficient for some cases. For example, if the ultimate goal is texture control of hot rolled steel, a higher sensitivity of measurement and response is needed. This is apparently due to lack of stability and offset and other errors of the amplifier. Also the various wires in the circuit may establish thermoelements which produce error voltages therein. Moreover, the photoelements are operated under load conditions in which the load current through the photoelements is not proportional to illumination density.